This invention relates to propellant systems and to burning rate modifiers for use therewith. In a more particular aspect, this invention concerns itself with solid rocket propellant compositions and to the use of transition metal fluorides as burning rate modifiers therefor.
A considerable increase in the utilization of propellant compositions has evolved with the present interest in the operation of rockets and guided missiles. As is well known, the primary object in using a propellant is to impart motion to an object through the mechanism of a combustion reaction which transforms the propellant into a gaseous form. The mechanism by which this is accomplished differs for the various classes and types of propellants. In liquid propellants, flow rates, vaporization rates, droplet size and formation are the important factors in the combustion reaction. For the solid propellant, the propellant composition, grain size, grain surface conditions and the mechanical structure of the propellant grain are of primary importance.
In the combustion of solid propellants, a significant characteristic is that the propellant grains burn in parallel layers. That is, the burning takes place in a direction perpendicular to the surface at all times. The rate at which the burning takes place is called the burning rate, and has the dimensions of velocity. In solid propellants, the burning rate is one of the most important factors to be considered and the design of any device which utilizes solid propellants is dependent upon a knowledge of this factor.
The burning rate of solid propellants is influenced by a number of factors including the pressure of the gas in contact with the burning surface. The rate increases with pressure which sometimes creates a problem in those areas, such as the nozzleless, motors that require a rather constant burning rate with changing chamber pressure. In those situations, it would be most desirable if one could produce a desirable higher burning together with a reduced pressure exponent. In attempting to overcome this problem, it was discovered that the use of transition metal fluorides as a burning rate modifier increased the burning rate of solid propellant formulations with a significant decrease in the pressure exponent. Heretofore, attempts at increasing the burning rate also increased the pressure exponent which severely limited the applicability of the propellant and the modifiers used to alter its burning rate.